When Dark Energy Turned On

MANCHESTER, UK — The Sloan Digital Sky Survey (SDSS-III) today announced the most
accurate measurements yet of the distances to galaxies in the faraway universe, giving an
unprecedented look at the time when the universe first began to expand at an ever-increasing
rate.

The results, announced today in six related papers posted to the arXiv preprint server,
are the culmination of more than two years of work by the team of scientists and
engineers behind the Baryon Oscillation Spectroscopic Survey (BOSS), one of the
SDSS-III's four component surveys.

The record of baryon acoustic oscillations (white rings) in galaxy maps helps
astronomers retrace the history of the expanding universe.

These schematic images show the universe at three different times. The false-color
image on the right shows the "cosmic microwave background," a record of what the
very young universe looked like, 13.7 billion years ago. The small density variations
present then have grown into the clusters, walls, and filaments of galaxies that
we see today. These variations included the signal of the original baryon acoustic
oscillations (white ring, right).

As the universe has expanded (middle and left), evidence of the baryon oscillations
has remained, visible in a "peak separation" between galaxies (the larger
white rings).

The SDSS-III results announced today (middle) are for galaxies 5.5 billion
light-years distant, at the time when dark energy turned on. Comparing them with
previous results from galaxies 3.8 billion light-years away (left) measures how
the universe has expanded with time.

"There's been a lot of talk about using galaxy maps to find out what's causing accelerating
expansion," says David Schlegel of the U.S. Department of Energy's Lawrence Berkeley National
Laboratory, the principal investigator of BOSS. "We've been making a map, and now we're using it
— starting to push our knowledge out to the distances when dark energy turned on."

"The result is phenomenal," says Will Percival, a professor at the University of Portsmouth
in the United Kingdom, and one of the leaders of the analysis team. "We have only one-third
of the data that BOSS will deliver, and that has already allowed us to measure how fast the
Universe was expanding six billion years ago — to an accuracy of two percent."

One of the most amazing discoveries of the last two decades in astronomy, recognized with
the 2011 Nobel Prize in Physics, was that not only is our universe expanding, but that
this expansion is accelerating — not only are galaxies are becoming farther apart
from each other, they are becoming farther apart faster and faster.

What could be the cause of this accelerating expansion? The leading contender is a strange
property of space dubbed "dark energy." Another explanation, considered possible but less
likely, is that at very large distances the force of gravity deviates from Einstein's
General Theory of Relativity and becomes repulsive.

Whether the answer to the puzzle of the accelerating universe is dark energy or modified
gravity, the first step to finding that answer is to measure accurate distances to as many
galaxies as possible. From those measurements, astronomers can trace out the history of
the universe's expansion.

BOSS is producing the most detailed map of the universe ever made, using a new
custom-designed spectrograph of the SDSS 2.5-meter telescope at Apache Point Observatory in
New Mexico. With this telescope and its new spectrograph, BOSS will measure spectra of
more than a million galaxies over six years. The maps analyzed in today's papers are based
on data from the first year and a half of observations, and contain more than 250,000
galaxies. Some of these galaxies are so distant that their light has traveled more
than six billion years to reach the earth — nearly half the age of the universe.

Maps of the universe like BOSS's show that galaxies and clusters of galaxies are
clumped together into walls and filaments, with giant voids between. These structures
grew out of subtle variations in density in the early universe, which bore the imprint
of "baryon acoustic oscillations" — pressure-driven (acoustic) waves that passed
through the early universe.

Billions of years later, the record of these waves can still be read in our universe.
"Because of the regularity of those ancient waves, there's a slightly increased probability
that any two galaxies today will be separated by about 500 million light-years,
rather than 400 million or 600 million," says Daniel Eisenstein of the Harvard-Smithsonian
Center for Astrophysics, director of SDSS-III and a pioneer in baryon oscillation surveys
for nearly a decade. In a graph of the number of galaxy pairs by separation distance, that
magic number of 500 million light years shows up as a peak, so astronomers often speak of
the "peak separation" between galaxies. The distance that corresponds to this peak
depends on the amount of dark energy in the Universe. But measuring the peak separation
between galaxies depends critically on having the right distances to the galaxies in
the first place.

That's where BOSS comes in. "We've detected the peak separation more clearly than ever
before," says Nikhil Padmanabhan of Yale University, who along with Percival co-chairs
the BOSS team's galaxy clustering group. "These measurements allow us to determine the
contents of the Universe with unprecedented accuracy."

In addition to providing highly accurate distance measurements, the BOSS data also
enable a stringent new test of General Relativity, explains Beth Reid, a NASA Hubble
Fellow at Lawrence Berkeley National Laboratory. "Since gravity attracts, galaxies at
the edges of galaxy clusters fall in toward the centers of the clusters," Reid
says. "General Relativity predicts just how fast they should be falling. If our
understanding of General Relativity is incomplete, we should be able to tell from
the shapes we see in BOSS's maps near known galaxy clusters."

Reid led the analysis of these "redshift space distortions" in BOSS. After accounting
for the effects of dark energy, Reid's team found that the rate at which galaxies fall
into clusters is consistent with Einstein's predictions. "We already knew that the
predictions of General Relativity are extremely accurate for distances within the solar
system," says Reid, "and now we can say that they are accurate for distances of 100
million light-years. We're looking a billion times further away than Einstein looked
when he tested his theory, but it still seems to work."

What's amazing about these results — six papers covering the measurements of cosmic
distance and the role of gravity in galaxy clustering -- is that they all come together to
tell the same story. "All the different lines of evidence point to the same explanation,"
says Ariel Sanchez, a research scientist at the Max Planck Institute for Extraterrestrial
Physics in Garching, Germany, and lead author on one of the papers. "Ordinary matter is
only a few percent of the universe. The largest component of the universe is dark energy —
an irreducible energy associated with space itself that is causing the expansion of the Universe
to accelerate."

But this is just the beginning, says BOSS principal investigator Schlegel. "For the past
13 years, we've had a simple model of how dark energy works. But the truth is, we only have
a little bit of data, and we're just beginning to explore the times when dark energy turned
on. If there are surprises lurking out there, we expect to find them."

About SDSS-III

Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the
Participating Institutions, the National Science Foundation, and the U.S. Department
of Energy Office of Science. The SDSS-III web site is
www.sdss3.org.

SDSS-III is managed by the Astrophysical Research Consortium for
the Participating Institutions of the SDSS-III Collaboration
including the University of Arizona, the Brazilian Participation
Group, Brookhaven National Laboratory, University of Cambridge,
Carnegie Mellon University, University of Florida, the French
Participation Group, the German Participation Group, Harvard
University, the Instituto de Astrofisica de Canarias, the Michigan
State/Notre Dame/JINA Participation Group, Johns Hopkins
University, Lawrence Berkeley National Laboratory, Max Planck
Institute for Astrophysics, Max Planck Institute for
Extraterrestrial Physics, New Mexico State University, New York
University, Ohio State University, Pennsylvania State University,
University of Portsmouth, Princeton University, the Spanish
Participation Group, University of Tokyo, University of Utah,
Vanderbilt University, University of Virginia, University of
Washington, and Yale University.